David LeBlanc has a really good article on the evolution of cryptography in Office.

David LeBlanc's Web Log : Office Crypto Follies

There have been a lot of advancements to this through the various Office releases but if you absolutely, positively need to lock down a document, I recommend using RMS, which will appear into the Office button menu as Restrict Permission.

In addition to encrypting the documents, RMS will also:

• Control access to sensitive info
• Set access level - view, change, print...
• Determine length of access
• Log and audit who has accessed rights-protected information

PS: I’m in Houston at the World Partner Conference. Come along to the Office Enterprise booth and say hello!